1) Field of the Invention
This invention relates to a polarization controlling apparatus and a polarization operation apparatus suitable for use in the fields of optical communication, optical signal processing, optical measurement and so forth, and more particularly to a polarization controlling apparatus and a polarization operation apparatus suitable for use as a component of a high-speed polarization scrambler in optical communication.
2) Description of the Related Art
A Faraday rotator is an optical device which rotates the polarization state of input light using a Faraday effect of rotating the polarization state of light using a magnetic field applied in the direction of an optical axis of the light. And the Faraday rotator is applied not only to optical isolators and circulators but also to polarization scramblers and like devices. Polarization scrambler to which the Faraday rotator is applied is demanded a performance of being variable the amount of rotation of the polarization state, and also it is demanded to achieve a higher-speed response.
Some conventional polarization controlling apparatus has a configuration such that a permanent magnet provides with a fixed magnetic field for biasing in order to minimize the loss of output light by formation of a single magnetic domain. However, a magnetic field generated by an electromagnet is progressively used directly as a magnetic field to be applied in the direction of an optical axis of light which has a direct relationship with the amount of rotation of the polarization state of light by a Faraday effect.
Also in techniques disclosed in Patent Documents 1 to 5 hereinafter mentioned, a magnetic field generated by an electromagnet exerts a Faraday effect directly on input light. Then, the current to a coil of the electromagnet is varied to vary the magnetic field to be generated by the electromagnet thereby to vary the magnitude of the Faraday effect to be applied to a ray of light which passes through the Faraday rotator.
Further, some known polarization scrambler which uses a Faraday rotation effect has a configuration wherein a single wave plate is interposed between two or four Faraday rotators (refer to Patent Document 3 or Patent document 4).
[Patent Document 1] Japanese Patent Publication No. Hei 6-9082
[Patent Document 2] Japanese Patent No. 2815509
[Patent Document 3] Japanese Patent Laid-Open No. Hei 9-61772
[Patent Document 4] Japanese Patent Laid-Open No. Hei 9-211405
[Patent Document 5] Japanese Patent Laid-Open No. 2005-99737
However, only if the magnetic field to be applied in the direction of an optical axis of light is varied by an electromagnet as described above, some restriction possibly appears to the degree of freedom in design of a polarization controlling apparatus because, when a configuration for application of a magnetic field is added to an optical system in designing of an apparatus, it is necessary for the optical axis to be set along the application direction of the magnetic field which is specified by the shape of the electromagnet.
Further, in order to increase the variation amount of the magnetic field of an electromagnet, it is a possible idea to increase the number of turns of a winding (coil) of the electromagnet or increase the value of the current to be supplied to the winding. However, if the number of turns of the winding is increased, then the inductance of the coil increases, which makes a high-speed operation difficult, and besides, increase of the apparatus scale cannot be avoided. On the other hand, if the current is increased, then since there is no necessity to increase the number of turns of the coil, increase of the inductance can be suppressed. Consequently, a high-speed operation is permitted. However, increase of the power dissipation cannot be avoided.
Accordingly, both of increase of the number of turns of the winding and increase of the current to be supplied to the winding have a factor of making an obstacle to the apparatus design. Also from this, it is considered that the degree of freedom in design of a polarization controlling apparatus may be subject to some restriction.
Further, such a polarization scrambler of a configuration wherein one wave plate is interposed between two or four Faraday rotators as disclosed in Patent Document 3 or 4 fails to sufficiently implement various polarization states.